The dominant problem associated with membrane bioreactors is membrane fouling, for which protein (PN) and polysaccharides (PS) are primarily responsible. However, little is currently known about these foulants. In the present study, combined influences of PN and PS on microfiltration (MF) have been investigated in terms of dead-end filtration mode. It revealed that the initial fouling rate of polyvinylidene fluoride (PVDF) was lower than that of track-etched polycarbonate (PC), except when the PN/PS had a ratio of 500:100 mg/L. Flux decline could be divided into two stages and a faster decline rate was assigned to a higher fouling rate. Resistance-in-series model was applied and the results showed that cake resistance rather than pore adsorption was the main fouling mechanism for both membranes during filtration with all combined ratios, and it would be easier caused at the lower ratios of PN/PS than the higher ratios. Moreover, pore plugging resistance increased in proportion to the increment of PN/PS ratio while maintaining constant PS concentrations. Backwashing removable fouling resistance was 69-97% and 85-97% for PVDF and PC membranes, respectively. Furthermore, adsorption resistance was higher for PC membranes, and both membrane materials showed better PS rejection than PN rejection. (C) 2010 Elsevier B.V. All rights reserved.